Method and a device for the combination of a first and second beam of rays

ABSTRACT

A method for combining a first and a second ray bundle includes forming a first partial beam and a second partial beam by the first ray bundle interacting with a first beam-splitter layer, and forming a third partial beam and a fourth partial beam by the second ray bundle interacting with the first beam-splitter layer. The first, second, third and fourth partial beams are superimposed into an outgoing ray bundle by the first, second, third and fourth partial beams interacting with a second beam-splitter layer so that, in the outgoing ray bundle, the number of occurring reflections and transmissions of the first and second partial beams are basically the same as the number of occurring reflections and transmissions of the third and fourth partial beams.

[0001] The present invention relates to a method and a device forcombining a first and a second ray bundle, in particular, in an opticalinstrument for forensic comparative examination of a first and a secondsample, according to the definition of the species in Claims 1 and 10.

[0002] When conducting forensic examinations to solve a crime, it isoften necessary to compare the image of a first sample to the image of asecond sample to obtain further details about the course of a crime.

[0003] Thus, for example, the impressions on cartridge cases caused bythe firing pins of a weapon are compared to each other to determinewhether the same weapon was used in two or more crimes.

[0004] Another known use of optical comparative examinations is toverify the authenticity of documents, in particular, of banknotes toascertain whether they are forged.

[0005] Finally, to solve crimes, it is often necessary to compare fibersfrom clothes found at the crime scene to fibers from known pieces ofclothing to be able to draw conclusions on the clothing worn by aperpetrator during a crime.

[0006] The applicant sells forensic comparison microscopes andcomparison macroscopes, which are composed of two separate microscopesor separate macroscopes joined by a bridge, and which are suitable forperforming the forensic comparative examinations mentioned above. Thebridge contains a device which is used to combine the two separateimages produced by the separate microscopes/macroscopes, and which isgenerally designed as a beam splitter known in optics for a long time.The two images of the separate microscopes/macroscopes to be comparedare combined by the beam splitter in such a manner that, when viewed bythe operator of the comparison microscope/macroscope through a sharedtube mounted on the bridge, the two images are perceived assuperimposed. By masking corresponding subareas of the two samples, acomposite image is formed which allows direct comparison, for example,of one half-sample to the other half-sample.

[0007] Beam splitters which are used for both splitting and combiningbeams of light are described, for example, in “Bauelemente der Optik”[Optical Components], Taschenbuch für Konstrukteure [Handbook forEngineers], 4^(th) revised edition, H. Naumann/G. Schröder, pp. 186through 188.

[0008] The known comparison microscopes/macroscopes have the problemthat, due to the design of the known beam-splitters in the tube, theperception of two separate images to be compared is not neutral incolor, which makes it considerably more difficult to compare the twosamples and, because of the missing or at least very limited colorinformation, also unnecessarily reduces the reliability with which twoprobes can be identified.

[0009] It is therefore an object of the present invention to provide amethod and a device which make it possible to produce an essentiallycolor-neutral, superimposed image when superimposing two separate imagesto be compared, in particular, in a comparison microscope or comparisonmacroscope for forensic examinations.

[0010] A further object of the present invention is to provide anoptical instrument for forensic comparative examination of a first and asecond sample which produces an essentially color-neutral, superimposedimage of the first and second sample.

[0011] This objective is achieved according to the present invention bythe features of Claims 1, 10, and 17.

[0012] Further features of the present invention are set forth in thesubclaims.

[0013] In a method according to the present invention for combining afirst and a second ray bundle, in particular, in an optical instrumentfor forensic examination of samples, the ray bundles are caused tointeract with a beam-splitter layer; each of the first and second raybundles both being reflected at the beam-splitter layer and passingtherethrough. Subsequently, the first and second partial beams resultingfrom transmission and reflection of the first and second ray bundles areeach caused to interact with a further beam-splitter layer such that thepartial beams of the first and second ray bundles resulting fromreflection and transmission at the beam-splitter layer and the furtherbeam-splitter layer interact once more with the further beam-splitterlayer before they are combined into an outgoing third ray bundle.

[0014] In the case of the first partial beam of the first ray bundle,this results in a reflection, followed by a transmission; and in thecase of the second partial beam, this results in an initialtransmission, followed by a reflection. In contrast, in the case of thefirst partial beam of the second ray bundle, first a reflection occurs,followed by a further reflections, and in the case of the second partialbeam, a transmission occurs, followed by a further transmission at thebeam-splitter layer or further beam-splitter layer. Accordingly, in theoutgoing third ray bundle, the sum of the reflections and transmissionsof the partial beams of the first ray bundle, which amount to a total of2 each, is equal to the sum of reflections and transmissions of thepartial beams of the second ray bundle.

[0015] Since, according to the present invention, the first and secondray bundles interact with the beam-splitter layer and the furtherbeam-splitter layer twice in succession before they are combined into anoutgoing third ray bundle, in the outgoing third ray bundle, the numberor sum of occurring reflections and transmissions of the first andsecond partial beams of the first ray bundle is essentially the same asthe number of occurring reflections and transmissions of the first andsecond partial beams in the incoming second ray bundle.

[0016] The method according to the present invention has the advantageof being almost free of color errors which, in the prior art beamsplitters having only one beam-splitter layer, for example, in the formof an interference layer or, in the simplest case, a semitransparentmirror, are caused by differences in the transmission and reflectioncoefficients of the beam-splitter layers and a physical wavelengthdependency thereof.

[0017] In the preferred embodiment of the present invention, thebeam-splitter layer has opposite first and second reflective surfacesrunning essentially parallel to each other; the first ray bundlestriking the first reflective surface at a first point of interaction,and the second ray bundle striking the second reflective surface at asecond point of interaction.

[0018] The beam-splitter layer is preferably a beam-splitter layer ofthe prior art which is produced in known manner and composed, forexample, of a plurality of metallic and dielectric films. The bodyelement is made, for example, of BAK 4® (a glass type of the SchottCompany); the first and second reflective surfaces being depositedthereon in the form of a metallic silver coating, for example, by vapordeposition. However, the beam-splitter layer can also be any otherbeam-splitter layer known from the prior art.

[0019] In order to obtain as symmetrical an arrangement as possible withthe smallest possible aberrations, the first point of interaction andthe second point of interaction are located substantially at the sameposition on opposite sides of the beam-splitter layer.

[0020] Similar to the beam-splitter layer, the further beam-splitterlayer also has opposite first and second reflective surfaces runningessentially parallel to each other. In this connection, the furtherbeam-splitter layer is preferably arranged in such a manner that thefirst partial beam of the first ray bundle resulting from reflectionstrikes the first reflective surface at a third point of interaction,and the second partial beam of the second ray bundle resulting fromreflection strikes the second reflective surface of the furtherbeam-splitter layer at a fourth point of interaction; the third point ofinteraction and the fourth point of interaction being locatedsubstantially at the same position on opposite sides of the furtherbeam-splitter layer in order to minimize aberrations.

[0021] In the preferred embodiment of the present invention, the firstpartial beam of the first ray bundle resulting from reflection at thebeam-splitter layer and the second partial beam of the second ray bundleresulting from transmission at the beam-splitter layer are deflectedtoward the third point of interaction by total reflection at a thirdreflective surface after interaction with the beam-splitter layer.

[0022] Preferably in the same way, the second partial beam of the firstray bundle resulting from transmission and the first partial beam of thesecond ray bundle resulting from reflection are deflected toward thefourth point of interaction, also by total reflection at a fourthreflective surface after interaction with the beam-splitter layer. Thethird and fourth reflective surfaces are preferably formed by themirrored lateral faces of a prism that run parallel to the beam-splitterlayer and to the further beam-splitter layer. Because of this, thereflection of the partial beams is essentially color-neutral compared toa reflection of the partial beams at a vapor deposition mirrorreflective surface, which would also be conceivable. The angle at whichthe aforementioned partial beams strike the third and fourth reflectivesurfaces, and preferably also the further beam-splitter layer, ispreferably around 45°, just as the angle of incidence of the first andsecond incident ray bundles with respect to the beam-splitter layer.This provides an optical path which is easy to implement and allowsimaging with very good color neutrality.

[0023] In the preferred embodiment of the present invention, both thereflectivity and transmissivity of the beam-splitter layer and of thefurther beam-splitter layer are essentially the same, resulting in acombination of ray bundles which has very good color neutrality. In thisconnection, it is particularly advantageous if the beam-splitter layerand the further beam-splitter layer are formed by one and the samebeam-splitter layer so that the incoming first and second ray bundlesand the partial beams resulting therefrom by transmission and reflectionafter the first interaction with the beam-splitter layer once moreinteract with the same beam-splitter layer are combined by the samebeam-splitter layer into the third, outgoing ray bundle at the fourthpoint of interaction. In this manner, it can be ensured that thereflectivity and transmissivity of the beam-splitter layers are largelythe same for the incoming ray bundles and for the partial beams, thuspreventing a reduction in color neutrality due to production tolerances.

[0024] If, due to production conditions, a beam-splitter layer having asplitting ratio of 50/50, has a transmissivity of, for example, 45% anda reflectivity of 55% for a specific wavelength, then the colorseparation of 45/55 of the prior art is reduced to 49.5/50.5 by themethod of the present invention.

[0025] This can be attributed to the fact that in the case of the firstincoming ray bundle, the reflected and transmitted portion contained inthe outgoing third ray bundle is 55%*45%+45%*55%=49.5% whereas thereflected and transmitted portion of the second incoming ray bundlecontained in the outgoing third ray bundle amounts to55%*55%+45%*45%=50.5%. Using the method according to the presentinvention, the color separation between the first and second incomingray bundles in the combined, outgoing third ray bundle can therefore beimproved by a factor of 10 compared to a prior art beam splitter havingonly one beam-splitter layer.

[0026] When speaking of “combining two ray bundles into a third,outgoing ray bundle” in connection with the method described above andthe device for carrying out the method which will be described below,the reverse beam path, in which an incoming third ray bundle is splitinto two outgoing first and second ray bundles, is also included. Such asplitting of a ray bundle into two further ray bundles is used, forexample, in microscopes to feed a partial image of the object that isvisible in the eyepiece to a digital image recording system fordocumentation purposes. The advantages of the present invention, inparticular, good color neutrality, also apply to these methods anddevices for splitting an incoming ray bundle or image into two outgoingray bundles (beam splitters).

[0027] In a further embodiment of the idea underlying the presentinvention, a device for combining a first and a second ray bundle orimage includes a beam-splitter layer with which the first and second raybundles interact, forming first and second partial beams, which aresuperimposed into a third, outgoing ray bundle. In addition to this, afurther beam-splitter layer is provided with which the first and secondpartial beams of the first and second ray bundles, upon interaction withthe (first) beam-splitter layer, interact once more such that, in theoutgoing third ray bundle, the number of occurring reflections andtransmissions of the first and second partial beams of the first andsecond ray bundles is essentially the same.

[0028] The beam-splitter layer and the further beam-splitter layer arepreferably designed as described in connection with the method of thepresent invention, and have opposite first and second reflectivesurfaces running essentially parallel to each other; the first raybundle striking the first reflective surface at a first point ofinteraction, and the second ray bundle striking the second reflectivesurface at a second point of interaction.

[0029] Similarly, the further beam-splitter layer has opposite first andsecond reflective surfaces running essentially parallel to each other,on which the first partial beam of the first ray bundle resulting fromreflection is incident at a third point of interaction, and the secondpartial beam of the second ray bundle resulting from reflection isincident at a fourth point of interaction on opposite sides.

[0030] According to a particularly advantageous embodiment of thepresent invention, the first partial beam of the first ray bundleresulting from reflection at the beam-splitter layer and the secondpartial beam of the second ray bundle resulting from transmissionthrough the beam-splitter layer are deflected toward the third point ofinteraction by total reflection at a third reflective surface afterinteraction with the beam-splitter layer.

[0031] Similarly, the second partial beam of the first ray bundleresulting from transmission at the first beam-splitter layer and thefirst partial beam of the second ray bundle resulting from reflectionare deflected toward the fourth point of interaction by total reflectionat a fourth reflective surface after interaction with the beam-splitterlayer. Due to the deflection of the partial beams by total reflection atthe third and fourth reflective surfaces, a beam splitter is providedthat is compact and inexpensive to manufacture, and which can be used,with good color neutrality, both to combine two ray bundles and to splitan incoming ray bundle into two outgoing ray bundles.

[0032] According to a preferred embodiment of the present invention, thebeam-splitter layer and the further beam-splitter layer are formed by asingle continuous beam-splitter layer. The device according to thepresent invention is preferably composed of two subprisms havingtrapezoidal bases and lateral faces which run at right angles to thetrapezoidal bases and are joined in the region of the long lateral face,preferably using optical cement. The beam-splitter layer and the furtherbeam-splitter layer are applied to the long lateral face of one of thesubprisms, for example, by vapor deposition or also with optical cement,prior to joining the two subprisms so that the (in this case) continuousbeam-splitter layer is located in the area of the junction of the twosubprisms. The subprisms are preferably made of glass, but can also bemanufactured from plastic or another suitable material.

[0033] In this embodiment of the present invention, the third and fourthreflective surfaces are each formed by the short lateral face which isopposite and parallel to the long lateral face of the respectivesubprism. The remaining two lateral faces are preferably arranged at anangle of 45° to the two lateral faces mentioned above. A first and asecond lateral face of these lateral faces arranged at an angle of 45°are used as surfaces of incidence for the first and second incoming raybundles, and a third lateral face constitutes an exit surface for thethird, outgoing ray bundle.

[0034] According to another embodiment, the remaining fourth lateralface of the subprisms, which is arranged at an angle, can be used as anexit surface for a further outgoing ray bundle which, for example, inthe case of comparison microscopes or comparison macroscopes, is fed viaa suitable imaging system to, in the simplest case, a still camera, butpreferably to a digital recording and processing system fordocumentation purposes.

[0035] Because of this, the device according to the present inventionadvantageously allows a further, essentially color-neutral compositeimage to be coupled out of the optical path for documentation purposeswithout the use of an additional beam splitter.

[0036] In a further embodiment of the idea underlying the presentinvention, the device described above is used in an optical instrumentfor forensic comparative examination of a first and a second sample, inwhich the inventive device receives a first image of the first sample inthe form of a first ray bundle through first imaging optics and a secondimage of the second sample in the form of a second ray bundle throughsecond imaging optics for combined representation.

[0037] According to the preferred embodiment of the present invention,the first imaging optics and the second imaging optics are formed by afirst and a second microscope, and the device for combining the twoimages is accommodated in a bridge joining the microscopes.

[0038] In this embodiment of the present invention, it can also beadvantageous if the bridge has mounted thereon and connected thereto ashared tube through which the combined first and second images can beviewed in the form of a third, outgoing ray bundle.

[0039] Finally, the optical instrument can contain a further imagingsystem which allows a further image, which is composed of the combinedfirst and second images, to be coupled out of the inventive device forcombining the first and second images in the form of a fourth ray bundlefor documentation purposes.

[0040] In the following, the present invention is described by way ofexamples with reference to the drawings, in which:

[0041]FIG. 1 shows an optical instrument according to the presentinvention in the form of a comparison microscope containing an inventivedevice for combining two ray bundles for forensic comparativeexamination of a first and a second sample; and

[0042]FIG. 2 is a schematic detail view of the inventive device forcombining a first and a second ray bundle into a third ray bundle and,in the case of a reverse optical path, for splitting a third, incomingray bundle into a first and second outgoing ray bundle.

[0043] As shown in FIG. 1, an optical instrument 1 according to thepresent invention for forensic comparative examination of a first sample2 and a second sample 4 includes first imaging optics in the form of afirst microscope 6 and second imaging optics in the form of a secondmicroscope 8, the microscopes being joined by a bridge 10 whichaccommodates an inventive device 12 which is also referred to as “beamsplitter” and combines a first image of first sample 2 in the form of afirst incoming ray bundle 14 and an image of second sample 4 which is inthe form of a second incoming ray bundle 16 and is to be compared to theimage of the image of the first sample; the inventive device 12 imagingthis combined image for combined representation in the form of a thirdray bundle 18 in a tube 20 mounted on bridge 10.

[0044] As further shown in FIG. 1, the optical instrument 1 according tothe present invention also includes a further, schematically shownoptical imaging system 22 which is composed of deflection mirrors (notfurther detailed) and allows a further image, which is composed of thecombined first and second images, to be coupled out of the device 12 forcombining the first and second images in the form of a fourth ray bundle24 for documentation with a camera 25; the deflection mirrors beingdrawn only schematically for reasons of representation.

[0045]FIG. 2 is a detail view of the device 12 according to the presentinvention for combining first ray bundle 14 and second ray bundle 18.

[0046] As can be seen from FIG. 2, the device 12 according to thepresent invention for combining first and second ray bundles 14, 16includes a first and a second subprism 26 and 28 between which isprovided an optical beam-splitter layer having a first reflectivesurface 36 and a second reflective surface 38; the beam-splitter layerbeing divided by imaginary dividing line 30 into a first beam-splitterlayer 32 and a further beam-splitter layer 34 for better understanding.

[0047] Upon entering the first subprism 26 through the substantiallyplanar entrance surface 40, the first incoming ray bundle 14 strikes thefirst reflective surface 36 of beam-splitter layer 32 at a first pointof interaction 42 where the first ray bundle 14 is split into a firstpartial beam 14 r resulting from reflection at first reflective surface36 and a second partial beam 14 t which results from transmission andpasses through beam-splitter layer 32.

[0048] Similarly, at the second reflective surface 38 of beam-splitterlayer 32, the second incoming ray bundle 16, which enters secondsubprism 28 through the substantially planar entrance surface 46, issplit at a second point of interaction 44 into a first partial beam 16 rresulting from reflection and a second partial beam 16 t resulting fromtransmission through beam-splitter layer 32.

[0049] Upon interaction with beam-splitter layer 32, the first partialbeam 14 r of first ray bundle 14 resulting from reflection and thesecond partial beam 16 t of second ray bundle 16 resulting fromtransmission are deflected, preferably by total reflection at a thirdreflective surface 48, toward a third point of interaction 49 where theysimultaneously strike first reflective layer 36 of further beam-splitterlayer 34; one part passing through the reflective layer and the otherpart being reflected. The portion of the two partial beams which isreflected at third point of interaction 49 constitutes a part of thefourth, outgoing ray bundle 24 which can be fed to a camera 25 fordocumentation purposes (FIG. 1).

[0050] Similarly, upon interaction with beam-splitter layer 32, thesecond partial beam 14 t of first ray bundle 14 resulting fromtransmission and the first partial beam 16 r of second ray bundle 16resulting from reflection are deflected by total reflection at a fourthreflective surface 50 toward a fourth point of interaction 52 where theysimultaneously strike second reflective layer 38 of furtherbeam-splitter layer 34; one part passing through the reflective layerand the other part being reflected. In the process, the portion of thetwo partial beams 16 r and 14 t which is reflected at fourth point ofinteraction 52 is superimposed with the portion of partial beams 14 rand 16 t which passed through the further beam-splitter layer 34 in theregion of third point of interaction 49 to form the outgoing third raybundle 18 which is fed to tube 20 as shown in FIG. 1.

[0051] Finally, the portion of the two partial beams 16 r and 14 t whichpassed through the further beam-splitter layer 34 at fourth point ofinteraction 52 is superimposed with the portion of partial beams 14 rand 16 t which is reflected in the region of third point of interaction49 to form the fourth, outgoing ray bundle 24.

[0052] List of Reference Numerals

[0053]1 optical instrument according to the present invention

[0054]2 first sample

[0055]4 second sample

[0056]6 first microscope

[0057]8 second microscope

[0058]10 bridge

[0059]12 device according to the present invention for combining two raybundles/beam splitter

[0060]14 first incoming ray bundle

[0061]14 r first partial beam obtained from the first ray bundle byreflection

[0062]14 t second partial beam obtained from the first ray bundle bytransmission

[0063]16 second incoming ray bundle

[0064]16 r first partial beam obtained from the second ray bundle byreflection

[0065]16 t second partial beam obtained from the second ray bundle bytransmission

[0066]18 third, outgoing ray bundle

[0067]20 tube

[0068]22 further optical imaging system

[0069]24 fourth ray bundle

[0070]25 camera

[0071]26 first subprism

[0072]28 second subprism

[0073]30 imaginary dividing line

[0074]32 beam-splitter layer

[0075]34 further beam-splitter layer

[0076]36 first reflective surface

[0077]38 second reflective surface

[0078]40 entrance surface for the first ray bundle

[0079]42 first point of interaction

[0080]44 second point of interaction

[0081]46 entrance surface for the second ray bundle

[0082]48 third reflective surface

[0083]49 third point of interaction

[0084]50 fourth reflective surface

[0085]52 fourth point of interaction

1-20. (canceled).
 21. A method for combining a first and a second raybundle, comprising: forming a first partial beam and a second partialbeam by the first ray bundle interacting with a first beam-splitterlayer; forming a third partial beam and a fourth partial beam by thesecond ray bundle interacting with the first beam-splitter layer; andsuperimposing the first, second, third and fourth partial beams into anoutgoing ray bundle by the first, second, third and fourth partial beamsinteracting with a second beam-splitter layer so that, in the outgoingray bundle, a number of occurring reflections and transmissions of thefirst and second partial beams are substantially the same as a number ofoccurring reflections and transmissions of the third and fourth partialbeams.
 22. The method as recited in claim 21 wherein the combining ofthe first and second ray bundles occurs in an optical instrument forforensic examination of specimens.
 23. The method as recited in claim 21wherein: the first beam-splitter layer has opposite first and secondreflective surfaces running substantially parallel to each other; andthe first ray bundle strikes the first reflective surface at a firstpoint of interaction, and the second ray bundle strikes the secondreflective surface at a second point of interaction.
 24. The method asrecited in claim 23 wherein the first point of interaction and thesecond point of interaction are disposed at a substantially samerelative position on opposite sides of the first beam-splitter layer.25. The method as recited in claim 21 wherein: the second beam-splitterlayer has opposite third and fourth reflective surfaces runningsubstantially parallel to each other; and the first partial beam resultsfrom reflection and strikes the third reflective surface at a thirdpoint of interaction; and the third partial beam results from reflectionand strikes the fourth reflective surface at a fourth point ofinteraction.
 26. The method as recited in claim 25 wherein the thirdpoint of interaction and the fourth point of interaction are disposed ata substantially same relative position on opposite sides of the secondbeam-splitter layer.
 27. The method as recited in claim 25 wherein thefourth partial beam results from transmission and further comprisingdeflecting the first partial beam and the fourth partial beam toward thethird point of interaction by total reflection at a fifth reflectivesurface after the respective interacting of the first and second raybundles with the first beam-splitter layer.
 28. The method as recited inclaim 25 wherein the second partial beam results from transmission andfurther comprising deflecting the third partial beam and the secondpartial beam toward the fourth point of interaction by total reflectionat a sixth reflective surface after the respective interacting of thefirst and second ray bundles with the first beam-splitter layer.
 29. Themethod as recited in claim 21 wherein a respective reflectivity of thefirst beam-splitter layer and of the second beam-splitter layer aresubstantially the same; and a respective transmissivity of the firstbeam-splitter layer and of the second beam-splitter layer aresubstantially the same.
 30. The method as recited in claim 21 whereinthe first beam-splitter layer and the second beam-splitter layerincludes a single continuous beam-splitter layer.
 31. A device forcombining a first and second incoming ray bundle, comprising: a firstbeam-splitter layer configured to receive a first and a second raybundle so that the first and second ray bundles interact therewith so asto form a first and a second partial beam by the first ray bundle and athird and a fourth partial beam by the second ray bundle; and a secondbeam-splitter layer configured to receive the first, second, third andfourth partial beams so that the first, second, third and fourth partialbeams interact so as to form an outgoing ray bundle via superposition ofthe first, second, third and fourth partial beams where, in the outgoingray bundle, a number of occurring reflections and transmissions of thefirst and second partial beams are substantially the same as a number ofoccurring reflections and transmissions of the third and fourth partialbeams.
 32. The device as recited in claim 31 wherein: the firstbeam-splitter layer includes oppositely disposed first and secondreflective surfaces running substantially parallel to each other; andthe first beam-splitter layer is configured so that the first ray bundlestrikes the first reflective surface at a first point of interaction andthe second ray bundle strikes the second reflective surface at a secondpoint of interaction.
 33. The device as recited in claim 31 wherein: thesecond beam-splitter layer includes oppositely-disposed third and fourthreflective surfaces running substantially parallel to each other; thefirst partial beam results from reflection and strikes the thirdreflective surface at a third point of interaction; and the thirdpartial beam results from reflection and strikes the fourth reflectivesurface at a fourth point of interaction.
 34. The device as recited inclaim 33 further comprising a fifth reflective surface configured todeflect the first partial beam and the fourth partial beam toward thethird point of interaction by total reflection after the respectiveinteracting of the first and second ray bundles with the firstbeam-splitter layer.
 35. The device as recited in claim 33 furthercomprising a sixth reflective surface configured to deflect the thirdpartial beam and the second partial beam toward the fourth point ofinteraction by total reflection after the respective interacting of thefirst and second ray bundles with the first beam-splitter layer.
 36. Thedevice as recited in claim 31 wherein the first beam-splitter layer andthe second beam-splitter layer include a single continuous beam-splitterlayer.
 37. The device as recited in claim 36 wherein the singlecontinuous beam-splitter layer includes an area of a junction of a firstand a second subprism, the first and second subprisms each having asubstantially trapezoidal cross-section.
 38. The device as recited inclaim 31 wherein the device is included in an optical instrument forforensic comparative examination of a first and a second sample viacombined representation of a first image of the first sample and asecond image of the second sample, the optical instrument furtherincluding first imaging optics configured to provide the first image asthe first ray bundle and second imaging optics configured to provide thesecond image as the second ray bundle.
 39. An optical instrument forforensic comparative examination of a first and a second sample viacombined representation of images of first and second samples,comprising: first imaging optics configured to provide a first image ofa first sample as a first ray bundle; second imaging optics configuredto provide a second image of a second sample as a second ray bundle; anda device for combining the first and second ray bundles, the deviceincluding: a first beam-splitter layer configured to receive the firstand second ray bundles so that the first and second ray bundles interacttherewith so as to form a first and a second partial beam by the firstray bundle and a third and a fourth partial beam by the second raybundle; and a second beam-splitter layer configured to receive thefirst, second, third and fourth partial beams so that the first, second,third and fourth partial beams interact so as to form an outgoing raybundle via superposition of the first, second, third and fourth partialbeams where, in the outgoing ray bundle, a number of occurringreflections and transmissions of the first and second partial beams aresubstantially the same as a number of occurring reflections andtransmissions of the third and fourth partial beams.
 40. The opticalinstrument as recited in claim 39 wherein the first imaging opticsincludes a first microscope and the second imaging optics includes asecond microscope, and further comprising a bridge configured to jointhe first and second microscopes, the device for combining beingdisposed on the bridge.
 41. The optical instrument as recited in claim40 further comprising a shared tube connected to the bridge, the firstand second images being perceivable by the human eye through the sharedtube as a combined image.
 42. The optical instrument as recited in claim39 further comprising an imaging system configured to couple out, fordocumentation purposes, a further image including a combination of thefirst and second images.